A lithographic apparatus is an apparatus that can be used in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred by a radiation beam via a projection system onto a target portion on a substrate, such as a silicon wafer. Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time. Known lithographic apparatus also include so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction while synchronously scanning the substrate parallel or anti parallel to this direction.
The cross-section of the radiation beam is typically much smaller than the surface of the substrate. So in order to expose all the target portions on the surface of the substrate, the substrate is moved relative to the projection system. The lithographic apparatus has a stage system to move the substrate relative to the projection system. The stage system is able to move the substrate in such a way that the target portions are placed consecutively in the path of the radiation beam.
To place each of the target portions in the path of the radiation beam correctly, the lithographic apparatus is provided with a position measurement system. The position measurement system measures a position of the stage system. The position measurement system comprises an interferometer and a reflective surface. Typically, the interferometer is stationary and the reflective surface is attached to the stage system. The interferometer is arranged to irradiate the reflective surface. The radiation that the reflective surface reflects back to the interferometer is used by the interferometer to generate a signal representative of a position of the reflective surface. Since the reflective surface is attached to the stage system, the signal is representative of a position of the stage system.